Abstract
The so-called chemical contribution to surface enhanced Raman scattering (SERS) of adsorbates on cold-deposited silver films in ultrahigh vacuum (UHV) is at least three orders of magnitude. The upper limit for classical enhancement, given by SERS of H 2O, is three orders of magnitude. Simultaneous SERS and DC-resistivity measurements demonstrate that the enhanced Raman scattering originates only from adsorbed molecules interacting with itinerant electrons of Fermi energy. Silver island films in UHV show a nonclassical first layer effect of, at least, a factor of 10, which is quenched by oxygen adsorption. Submonolayer postdeposition of silver on adsorbates on smooth silver induces chemically specific and vibrationally selective Raman enhancement. Atomic scale roughness induces enhanced Raman scattering. The results are tentatively explained by an electronic enhancement mechanism based on inelastic scattering of intermediate hot electrons by adsorbate vibrations via shape resonances. Problems and ideas on the future of the SERS spectroscopy are pointed out.
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